The technologies of the commercial solid-state lasers are mostly bulk-optic. A bulk crystal doped with active doping ions is used as the gain medium. Considering pump absorption of the gain medium with a limited length in a laser cavity, a higher doping concentration is necessary. However, adverse effects including the thermal loading, quenching, and energy transfer up-conversion (ETU) arise when the doping concentration becomes higher. As a consequence the optical efficiency degrades.
In the prior art, such as U.S. Pat. No. 4,815,079, for example, a double cladding optical fiber for the high power optical fiber laser and amplifier applications using the cladding pump method is described. However, the double-clad optical fibers fabricated in accordance with prior art are all made of glass.
Furthermore, the single-crystal fibers are fabricated by means of Laser-Heated Pedestal Growth (LHPG) method or micro-pulling down method using bulk crystals as the source rods for the use of laser gain media. However, the diameter of a single-crystal fiber is pretty large, e.g., several hundreds of micrometers. Also the single-crystal fiber is un-clad so it is not suitable for high power fiber laser applications using the cladding pump method.
Further, regarding the propagation loss of inner cladding layer, in the prior art, such as U.S. Pat. No. 7,333,263 B2, a fabrication process of a double-clad crystal fiber is described therein, in which, however, the inner cladding layer is made of the mixture of crystal and glass, having numerous nanoscale crystalline particles therein, thus causing light scattering. Moreover, the doping ions in the nano particles are capable of absorbing pumping light, leading to the absorption loss of pumping light in the inner cladding region. Furthermore, this double cladding crystal fiber is, in general, merely suitable for lower power laser using the core-pumped configuration. If it is further used in high power laser of the cladding-pumped configuration, the nano particles in and the doping ions therein cause considerable scattering and absorption of pumping light, in such a way that the optical efficiency of this double cladding crystal fiber using cladding-pumped configuration is impaired significantly.